Adobe recommends

TrilemetryAdobe TV hosts a Flex in a Week
training video that introduces the concepts of styling and skinning.

Flex 4 deemphasizes the use of style properties for some tasks.
Instead of styling individual components, or classes of components,
you can use custom skins to perform most operations that change
the look and feel of your applications. This includes adding background
colors and images, borders, and other properties that were styleable
in Flex 3 applications. For more information on skinning, see Spark Skinning.

Peter deHaan’s blog, FlexExamples.com has many examples of
using styles and themes in Flex.